Laser-Ultrasonics as a Technique to Study Recrystallisation and Grain Growth

Laser-Ultrasonics (LUS) provides a means of obtaining microstructure information continuously and non-destructively both in the laboratory and for quality control on-line in industry. Ultrasound is both generated and recorded using lasers which permits remote, non-contact operation with fast sampling and also the capability of working at high temperatures or at moving surfaces, for example during industrial continuous annealing. Examples of dynamic heating trials will be presented for samples of cold rolled steel sheets where primary recrystallisation and ferrite austenite transformation are monitored in-situ as a function of temperature. Examples are also presented where the grain size of low carbon steels have been quantitatively analysed and show very good agreement with microscopy methods.

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Grain Refinement by Rapid Transformation Annealing of Cold Rolled Low Carbon Steels

Materials Science Forum ◽

10.4028/www.scientific.net/msf.500-501.771 ◽

2005 ◽

Vol 500-501 ◽

pp. 771-778 ◽

Cited By ~ 8

Author(s):

P. Álvarez ◽

C. Lesch ◽

Wolfgang Bleck ◽

Hélène Petitgand ◽

Joachim Schöttler ◽

...

Keyword(s):

Grain Refinement ◽

Rapid Heating ◽

Carbon Steels ◽

Low Carbon ◽

Steel Sheets ◽

Hsla Steels ◽

Mn Content ◽

Cold Rolled ◽

Equiaxed Grains ◽

Rapid Transformation

A novel thermal treatment, rapid transformation annealing (RTA), has been applied to six different cold rolled low-carbon (LC) steel sheets with the aim of refining their microstructure. The process involves rapid heating to just above the austenite (g) to ferrite (a) transformation temperature and subsequent rapid cooling to room temperature. Grain sizes around 2 µm in two different Nb-Ti HSLA steels, 5 µm in a Ti-LC steel and 6 µm in a plain LC (0.037%C) steel have been produced using fast cooling rates (200°C/s). Non-equiaxed structures are obtained in a Nb-Ti HSIF steel and in a plain LC (0.135%C) (CM) steel due to their higher Mn content. However, very fine equiaxed grains (2 µm) are obtained by rapid intercritical annealing (RIA) in the CM steel. Irrespective of the microalloying concept, the grain growth of recrystallized a grains before their transformation was inhibited in CM and in both HSLA steels. This inhibition is connected with the overlapping of a recrystallization and a-g transformation processes which is essential in order to achieve extreme grain refinement either by RTA or RIA.

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Recrystallization of Boron Containing and Boron-Free Low Carbon Steels Investigated by Thermoelectric Power and Hardness Measurements

Materials Science Forum ◽

10.4028/www.scientific.net/msf.812.195 ◽

2015 ◽

Vol 812 ◽

pp. 195-199

Author(s):

András Mucsi ◽

László Dévényi

Keyword(s):

Thermoelectric Power ◽

Carbon Steels ◽

Low Carbon ◽

Slow Cooling ◽

Steel Sheets ◽

Starting Point ◽

Different Temperatures ◽

Good Starting Point ◽

Cold Rolled ◽

Metallurgical Processes

The measurement of thermoelectric power is a powerful method to investigate the metallurgical processes occurring in steels. In this study, four low carbon cold rolled steel sheets having different composition were investigated. The cold rolled sheets were heated up at heating rate 20 °C/hour up to different temperatures, whilst the change of thermoelectric power and hardness have been measured at room temperature after slow cooling. The thermoelectric power of steels increases with temperature until the recrystallization finishes. The raise of thermoelectric power during recrystallization is ranging between 50 and 160 nV/K, depends on the composition of the sheet. Specimens consist of boron exhibit lower thermoelectric power after recrystallization than the steel without any boron, probably due to metastable borocarbide dissolution. This fact could be a good starting point to investigate the effect of boron on thermoelectric power of steels.

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Cold Rolled Microstructure and Its Evolution during Recovery and First Recrystallisation Stages in Low Carbon Steels

Materials Science Forum ◽

10.4028/www.scientific.net/msf.467-470.241 ◽

2004 ◽

Vol 467-470 ◽

pp. 241-246 ◽

Cited By ~ 2

Author(s):

M. Díaz-Fuentes ◽

E. Novillo ◽

Amaia Iza-Mendia ◽

Isabel Gutiérrez

Keyword(s):

Deformation Texture ◽

Carbon Steels ◽

Subgrain Structure ◽

Processing Conditions ◽

Low Carbon ◽

Electron Backscattered Diffraction ◽

Steel Sheets ◽

Deformation Banding ◽

Different Temperatures ◽

Cold Rolled

During annealing of cold rolled steel sheets, the evolution of the texture, to produce the characteristic texture of a recrystallised material, depends on a series of factors, including the starting deformation texture, composition and processing conditions. In the present work, electron backscattered diffraction (EBSD) techniques have been used to investigate the deformation substructures being developed by cold rolling into grains belonging to different texture components. The strain localisation, the deformation banding, the spread in orientation into grains with different orientations are some of the aspects that have been considered. Annealing cycles at different temperatures have also been carried out in order to promote recovery and initial stages of recrystallisation . The obtained substructures have been compared to the as-cold rolled ones. It has been observed that recovery induces the dislocation structures to arrange into subgrains. Associated to the ND fibre, a network of relatively high angle boundaries develop within the subgrain structure. The recrystallisation nuclei have been observed to evolve from such a network.

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Laser-Ultrasonic Austenite Grain Size Measurements in Low-Carbon Steels

Materials Science Forum ◽

10.4028/www.scientific.net/msf.715-716.407 ◽

2012 ◽

Vol 715-716 ◽

pp. 407-414

Author(s):

Matthias Militzer ◽

Mehran Maalekian ◽

André Moreau

Keyword(s):

Grain Size ◽

Carbon Steels ◽

Laser Ultrasonics ◽

Mechanical Processing ◽

Low Carbon ◽

Laser Ultrasonic ◽

Austenite Grain Size ◽

Low Carbon Steels ◽

Austenite Grain

Austenite grain size is an important microstructure parameter when processing steels as it provides the initial condition for the austenite decomposition that determines the final microstructure and thus properties of the steel. In low-carbon steels it is frequently difficult if not impossible to quantify the austenite grain size using conventional metallographic techniques. Laser-ultrasonics provides an attractive alternative to quantify the grain size in-situ during thermo-mechanical processing of a steel sample. The attenuation of the laser generated ultrasound wave is a function of the grain size. The present paper gives an overview on the state-of-the-art of this novel measurement technique. Using isothermal and non-isothermal grain growth tests in low-carbon steels the advantages and limitations of laser-ultrasonic measurements will be demonstrated. Further, their application for deformed samples will be presented to quantify austenite grain sizes during and after recrystallization. The in-situ measurements provide significantly new insights into the austenite microstructure evolution during thermo-mechanical processing of low-carbon steels. The implications on expediting the development of improved process models will be discussed.

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Model for Predicting Recrystallization Behavior of Cold Rolled Extralow Carbon Steel Sheets

Materials Science Forum ◽

10.4028/www.scientific.net/msf.706-709.2302 ◽

2012 ◽

Vol 706-709 ◽

pp. 2302-2307 ◽

Cited By ~ 2

Author(s):

Takehide Senuma ◽

Yoshito Takemoto

Keyword(s):

Carbon Steel ◽

Steel Sheet ◽

Carbon Steels ◽

Recrystallization Behavior ◽

Abnormal Growth ◽

Steel Sheets ◽

Cold Rolled ◽

Carbon Steel Sheet ◽

Good Agreement

Microstructual observations indicate that the recrystallization of cold rolled extralow carbon steel sheets occurs due to the abnormal growth of selected subgrains in recovered subgrain microstructures. The authors measured the orientations of a recovered microstructure of a cold rolled extralow carbon steel sheet by SEM-EBSP and classified the deformed grains into several types due to the orientation and its scattering degree. In this study, a model for predicting the recrystallization behavior of cold rolled extralow carbon steels has been developed by applying the model of Humphreys modified to the grain of each type.A good agreement between the experimental and calculated results using the model developed was obtained.

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In Situ Measurements of Grain Growth and Recrystallization by Laser Ultrasonics

Materials Science Forum ◽

10.4028/www.scientific.net/msf.753.25 ◽

2013 ◽

Vol 753 ◽

pp. 25-30 ◽

Cited By ~ 10

Author(s):

Matthias Militzer ◽

Thomas Garcin ◽

Warren J. Poole

Keyword(s):

Grain Growth ◽

Thermomechanical Processing ◽

Texture Evolution ◽

Carbon Steels ◽

Laser Ultrasonics ◽

Sensor Technology ◽

Austenite Formation ◽

Low Carbon ◽

Potential Applications

Laser ultrasonics for metallurgy (LUMet) is an innovative sensor technology for in-situ measurement of microstructure evolution during thermomechanical processing. This unique sensor has been attached to a Gleeble 3500 thermomechanical simulator for dedicated laboratory studies during processing of steel, aluminum, magnesium and titanium samples. Advanced processing software has been developed for the measurement of grain size and texture evolution from laser ultrasonic signals. Results of austenite grain growth measurements in low carbon steels will be described to demonstrate the capabilities of the LUMet technique. Further, applications of the system to measure recrystallization of ferrite and austenite formation during intercritical annealing simulations of dual phase steels will be presented. The ability to rapidly acquire data both during a single test and for multiple conditions over a range of conditions from different samples has important implications on expediting process modelling and alloy design. Although certain limitations exist, the LUMet technique offers a very reliable characterization platform with a number of potential applications in metallurgical process engineering.

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